Many printer and additive manufacturing embodiments use printed test patterns to perform marking device alignment and registration processes during operation. As used herein, the term “test pattern” refers to a predetermined arrangement of marks that are formed by depositing printing material from one or more marking devices on an image receiving surface. A printer receives digital image data that correspond to the test pattern and selectively deposits marking material on an image receiving surface. When the marking device is an inkjet, the selective deposition is performed by operating a portion of the ejectors in the printheads to form the printed test pattern on an image receiving surface, such as a paper print medium or an indirect support member that receives the marking agent. The marks in the test pattern are separated from each other to enable an optical sensor to generate scanned image data of the printed test pattern. A controller in the printer processes the printed marks in the scanned image data to, for example, identify the location of a marking device in the print zone and identify relative registration errors between multiple marking devices. The registration errors can negatively impact the quality of printed documents and manufactured parts, and some printer embodiments identify and correct the registration errors during a job to maintain the visual quality of printed documents and the functionality of printed parts.
As used herein, the term “disjoint test pattern” refers to a test pattern that a printer forms where multiple marks separated from each other are created from a single marking device. Typically, the marks formed are a small percentage of the total marks that a particular marking device can make. The particular arrangement of marks in the disjoint test pattern created from each marking device differs from any of the other marking devices.
As used herein, the term “sparse test pattern” refers to a disjoint test pattern that a printer forms in a configuration that is difficult for human viewers to perceive. For example, traditional test patterns include comparatively large marks that each ejector in a printhead forms from several drops of a marking agent, such as ink. The test patterns are arranged in a regular series of rows that are easily visible to an optical sensor and to an average human observer. In some printer embodiments, a sparse test pattern is formed in margins, which are also referred to as “inter document zone” between printed pages during a multi-page print job. A sparse test pattern, however, includes marks that are formed from a smaller number of drops of the marking agent, and often from only a single drop. Additionally, the sparse test patterns include marks that are formed with an irregular pattern that reduces or eliminates the visibility of the sparse test pattern to a human observer.
One drawback to using disjoint test patterns during operations in the printer is that the contrast between the ink and the media may be small and thus it becomes difficult to sense the location of the individual marks created with the disjoint test pattern. One drawback to using sparse test patterns during operations in the printer is that the reduced visibility of the sparse test pattern also makes identification of the individual printed marks in the test pattern more difficult for a controller in the printer. While optical sensors can generate image data with sufficient quality to resolve printed marks in a sparse test pattern, under some conditions, small artifacts on the image receiving surface, such as paper fibers or contaminants, produce visual noise that can appear similar to the printed marks in the sparse test pattern. For these cases, the controller incorrectly identifies an artifact in the noisy image data as one of the marks in the test pattern, which reduces the accuracy of the printhead registration process. Consequently, improved systems and methods for identifying disjoint test patterns during printhead registration operations would be beneficial.